In a clutch/brake of an automatic transmission, a multi-disc frictional engagement mechanism is used to couple two members (both are rotational elements in the case of the clutch, one is a rotational element and the other is a non-rotational element in the case of the brake) which are coaxially arranged.
In the multi-disc frictional engagement mechanism, a plurality of friction plates are mounted slidably in an axial direction on the respective two members and the friction plates of the two members are alternately arranged. When the friction plates of the two members are pressed against each other by a hydraulic piston, the two members are coupled via the friction plates.